An X-ray CT apparatus is an image diagnosis apparatus that irradiates, mainly, radiates an X-ray, detects transmission thereof through a subject, and reconstructs an image of the inside of the subject from projection data indicating the intensity of the radiation having been detected. The apparatus plays an important role in many medical practices including diagnosis of a disease and plan of treatment and operation. The X-ray CT apparatus irradiates a subject from multiple directions, and reconstructs one tomographic image from, for example, projection data for 360 degrees or projection data for 180 degrees plus a fan angle (the angle of a fan beam). Moreover, the apparatus reconstructs a volume image, which is a three-dimensional image, from projection data stacked by continuously moving a scan position in the direction of the body axis of the subject.
In recent years, as a method for imaging a subject with the X-ray CT apparatus, various methods have been proposed. One example of the methods is a real prep scan. In the real prep scan, the flow of a contrast agent into a slice of interest is detected in a prep scan at low doses and, when a sufficient amount of contrast agent flows into a region of interest, the scan is switched to a real scan. Another example of the methods is an ECG-gated scan. In the ECG-gated scan, electrocardiographic waveforms are acquired by an electrocardiograph and a scan aspect is changed in synchronization with appearance of a predetermined cardiac phase. Moreover, there is a method of changing a scan aspect for each site when collectively scanning a plurality of sites.
The scan aspects are all for switching imaging conditions during a scan. For example, the X-ray dose is switched between a prep scan and a real scan, or the X-ray dose is increased when a predetermined cardiac phase appears, or a tube current supplied to a coil filament of an X-ray tube is switched for each site.
In conventional techniques, an X-ray tube generates an X-ray by, in a state that a coil filament and an anode face each other, supplying a tube current and applying a high voltage to the coil filament so that thermo electrons are generated and radiated to the anode. This X-ray tube is generally provided with a large filament with a large number of windings and a small filament with a small number of windings.
With the small filament, it is possible to obtain a high-resolution image because the focal point of electrons radiated to the anode is small in size, but it takes time to scan. On the other hand, with the large filament, because the focal point of electrons radiated to the anode is large in size, it is possible to radiate an X-ray to a wide range at one time and scan in a shorter time, and it is also possible to obtain a low-noise image. However, it is impossible to obtain a high-resolution image. Thus, the X-ray tube is provided with the small filament and the large filament so that these two filaments are selectively used.
For switching from the small filament to the large filament or vice versa, that is, for changing a filament to which a tube current is supplied and a tube voltage is applied, there is a process of turning off supply of the tube current and application of the tube voltage to one of the filaments and then turning on supply of the tube current and application of the tube voltage to the other filament. Consequently, it takes much time to switch the filaments.